English grammar: Modal verbs for obligation

What this grammar point covers

Modal verbs for obligation are words like 'must' and 'have to' that show when something is necessary or required in English.

When to use it

Use these forms when you want to say that something is necessary, required, or an obligation. For example, rules, laws, or important tasks.

Key forms

• 'must' + base verb (e.g. must go)
• 'have to' + base verb (e.g. have to work)
• Negative: 'must not' (mustn't), 'don't have to'

Examples

Tips

• Use 'must' for strong personal obligation or rules.
• 'Have to' is used for general obligations, often from outside (like laws or schedules).
• 'Must not' means it is not allowed; 'don't have to' means it is not necessary.

Exceptions and edge cases

• 'Must' does not have a past form. Use 'had to' for past obligations.
• In questions and negatives, use 'do/does' with 'have to' (e.g. Do you have to leave?).